Critical Fluctuations in the Microwave Complex Conductivity of BSCCO and YBCO Thin Films

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Critical fluctuations above Tc are studied in the microwave complex conductivity of BSCCO-2212, BSCCO-2223, and YBCO thin films. The analysis of the experimental data yields the temperature dependences of the reduced coherence length ξ(T)/ξ₀. Two critical regimes are observed, having the static critical exponent ν=1 close to Tc, and a crosover to ν=2/3 at higher temperatures. In more anisotropic superconductors the reduced coherence lengths are larger, and the critical states extend to higher temperatures.